A light string includes a plurality of light sources directly soldered onto an electric cord at intervals, so as to form a string-shaped illumination device without a lamp holder in the art. To small-sized light sources, such as small bulbs, light emitting diodes (LEDs), light strings are a common arrangement of the light sources. A light string is as flexible as the electric cord is, such that the light string is easily arranged in any configuration to comply with requirements for special illumination or decoration.
A known simple light string is that is connected to a plurality of light sources in parallel among two power wires. In this parallel circuit, the driving voltage difference is required to drive individual light sources, which is different than in a series circuit, as the driving voltage has to be raised when the number of the light sources is increased. Meanwhile, in the parallel circuit, individual failed light source will not cut off the power loop of the other light sources.
However, when various types of LEDs are simultaneously used as the light sources, various turn-on voltages exist. When LEDs of various colors, or shades of colors, are simultaneously used, ratios of brightness of these colors have to be proper determined to achieve preferable visual effects. However in a parallel circuit, an LED with a low turn-on voltage could lower down the driving voltage difference among two power wires, such that the voltage difference between two ends of a high turn-on voltage LED is too low and any high turn-on voltage LEDs will not be turned-on or will emit light with insufficient brightness. If the driving voltage difference is raised to drive the high turn-on voltage LEDs, the brightness of the low turn-on voltage LEDs could be too high, thereby causing the low turn-on voltage LEDs to fail. Therefore, when LEDs of various colors are simultaneously used, a plurality of circuits are required to drive LEDs with different colors.